In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least two layers: an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
The imagewise discharge is brought about by the radiation-induced generation of electron-hole pairs, by a material (often referred to as a charge-generation material) in the electrophotographic element. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface in the exposed areas and cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image. Electrophotographic elements often have separate layers that can be identified as a charge generation layer (CGL) and a charge transport layer (CTL) on the basis of their primary functions.
Many electrophotographic elements are designed to be initially charged with a negative polarity. They contain material, known as a hole-transport agent, which facilitates the migration of positive holes toward the negatively charged surface in imagewise exposed areas. A positively charged toner is used to develop the unexposed areas. Because of the wide use of negatively charging elements, many types of positively charging toners are available.
For some applications, however, it is desirable to develop the exposed rather than the unexposed surface areas of the element. For example, in laser printing of alphanumeric characters it is more desirable to expose the small surface area that will form visible alphanumeric toner images, rather than waste energy exposing the large background area. In order to accomplish this with available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Positive toner can then develop the exposed surface areas (which will have relatively negative electrostatic potential).
An electrophotographic element designed to be initially positively charged should contain an electron-transport agent, that is, a material which facilitates the migration of photogenerated electrons toward the positively charged surface. Unfortunately, while many good hole-transport agents are available, relatively few electron transport agents are known and many prior art compounds have one or more drawbacks.
In order for electron transport to occur in the CTL, two events are necessary. The first event is the capture of the electron injected by the CGL. The second event is the movement of electrons from one molecule to the next in the CTL. The former process affects the quantum yield for electron capture as measured in terms of electrophotographic speed under conditions of low-intensity continuous exposure, while the latter process represents the kinetics of electron exchange in the CTL as measured in terms of mobility.
Some previous electron-transport agents do not perform the electron transporting function well except under limited conditions or in limited types of electrophotographic elements. Some agents cause an undesirably high rate of discharge of the electrophotographic element before it is exposed, also referred to as high dark decay.
Some previous electron-transport compounds have limited solubility or dispersibility in coating solvents and limited compatibility in polymeric binders. Increasing the concentration of an electron-transport agent in a polymeric layer, in the absence of phase-separation, increases the electron-transport mobility of the layer; accordingly, photogenerated electrons move through the layer at a higher velocity and traverse the layer in a shorter period of time. The higher the mobility, the shorter is the waiting period between exposure and development, and the greater is the number of copies that can be made in a given amount of time. Even when sufficient amounts of electron-transport agent can be compatibly incorporated in an electrophotographic element during manufacture, problems can arise during use due to migration of the electron transport agent.
Among electron-transport materials, those molecules based on 1,1-dioxo-4H-4-(dicyanomethylidene) thiapyran-4-one (the "sulfones") have electron mobilities and electrophotographic speeds that are high relative to most electron-transport materials. U.S. Pat. Nos. 4,514,481 to Scozzafava et al; and 4,968,813; 5,013,849; 5,034,293; and 5,039,585 all to Rule et al; teach derivatives of 4H-thiopyran-1,1-dioxides. U.S. Pat. No. 4,514,481 discloses 4-dicyanomethylene-2,6-diphenyl-4H-thiopyran-1,1-dioxide (also referred to herein as DPS), which has the structural formula: ##STR2##
U.S. Pat. No. 5,039,585 discloses 4-dicyanomethylene-2-p-tolyl-6-phenyl-4H- thiopyran-1,1-dioxide (also referred to herein as PPS), which has the structural formula: ##STR3## DPS and PTS have about the same electron mobilities, however, PTS is more soluble.
The terms "solubility" and "compatibility" are used herein to describe the ability of a first material to disperse in a second material so as to form a homogenous blend. The term "solubility" is generally used to describe the ability of a material to dissolve in liquid solvents to form a solution. The term "solubility" used herein, does not, however, exclude dispersions which appear to be homogeneous, at least to unmagnified examination. The term "compatibility" is generally used to describe the ability of a material to blend with a polymer so as to produce a material which appears to be homogeneous, at least to unmagnified examination. The term "miscibility" defines a similar concept. The terms "incompatible" and "incompatibility" and the like, used herein refer to an intermixture which is characterized by segregation of sulfone and polymer binder, that is, crystallization or aggregate formation which is visible without magnification. Such crystallization or aggregate formation causes such problems as undesirable dark decay, as well as scatter or absorption of actinic radiation intended to pass through the charge-transport layer.
U.S. Pat. No. 4,514,481 describes the incorporation of DPS and other similar sulfones in polymeric binder layers of electrophotographic elements at a concentration of 30% by weight (based upon the total weight of the agent and the binder) for good performance. The upper limit of compatibility (solubility or homogeneous dispersibility) of compounds such as DPS in many polymeric binders is about 40% by weight. At such concentrations, DPS and similar sulfones are on the edge of incompatibility and an elevation in temperature can cause migration within the binder and the formation of undesirable crystalline aggregates. This is a major shortcoming, since electrophotographic elements encounter elevated temperatures during normal use in a copier. PTS has a higher solubility than DPS in solvents useful for preparing a photoconductor. This allows higher loading levels in the final photoconductor composite, which in turn results in increased photogenerated charge. migration, leading to potentially faster photodischarge speeds and lower toe/erase voltages. PTS has a compatibility limit of about 60% by weight. At that loading level, PTS has crystallization problems comparable to the problems seen with DPS at 40% loading.
Thus, there is a need for electrophotographic elements containing electron transport agents which have good electron mobilities and electrophotographic speeds and exhibit good solubility in coating solvents and good compatibility with polymeric film-forming binders and thus can be used at very high loading levels.